This invention relates generally to a method of operating an electron capture detector (ECD) which provides a greatly increased linear dynamic range and more particularly to an ECD designed for application of bipolar pulses to enhance electron extraction at high frequencies.
By the electron capture detector technique in gas chromatography, a radiation source such as a radioactive foil ionizes the molecules of a carrier or make-up gas as it flows through the detector and the slow electrons thus produced are caused to migrate to the anode, forming a steady or pulsed current. This detector current becomes reduced if a sample containing electron-absorbing molecules is introduced and this loss of current is amplified by an electrometer and is analyzed.
Since its first introduction in 1960, the ECD has claimed advantages in its high specificity and sensitivity, but they have equally been plaqued by the lack of linear dynamic range. A new mode of operation for the ECD was proposed by Maggs, et al. (Analytical Chemistry, 43 1966 (1971)) according to which a constant current is maintained by changing the frequency of applied pulses, the theory being that the change in pulse frequency is proportional to the concentration of the electron-capturing species present within the ECD. In practice, however, the constant current mode is non-linear at frequencies exceeding 80 KHz. This may be due to collection of anions as well as electrons near the collector. Effects of applied fields on positive ions in the pulsed ECD have been considered generally, for example, by Connolly, et al, (J. of Chromatography, 265 145 (1983)).